I have some CDs that I'm not sure if I burned them from mp3 files or if I made a 1 to 1 CD-copy. Is there a way to find out if the source was mp3 or CD? I have the original CDs as well, ripped to FLAC, for reference.

If you also have the original CDs in FLAC and just want to compare the copy to those, then the easiest way is to rip one track from the copy to FLAC (or WAV), open both FLACs in some audio editor and compare the frequency spectrums (there is a tutorial somewhere here on this topic). If the copy was made from mp3, there will be noticable differences in the high frequencies. If the copy was made as a 1:1 image, they will be identical.